Device for preventing a corona discharge from assuming the form of a spark discharge in electrostatic painting apparatus

ABSTRACT

A protective device preventing the transformation of a corona discharge into a spark discharge in electrical apparatus, particularly, in electrostatic painting apparatus, consists of a multielectrode gas discharge tube having a control grid which through a starting unit is connected to pickups or sensing elements controlling the operating current. The multielectrode gas discharge tube which may be a thyratron has a keep-live electrode. A comparison unit having a source of reference voltage is connected through a contact of a time relay to the keep-live electrode.

United States Patent [191 Sokolsky et al.

[54] DEVICE FOR PREVENTING A CORONA DISCHARGE FROM ASSUMING THE FORM OF A SPARK DISCHARGE IN ELECTROSTATIC PAINTING APPARATUS [76] Inventors: Valery Nikolaevich Sokolsky,

Pesochnaya, 2, ultisa Leningradskaya, 70/2, kv. 8; Igor Ivanovich Mamaev, Mitavsky pereulock, 3, kv. 26, both of Leningrad, U.S.S.R.

[22] Filed: Jan. 28, 1972 [21] Appl. No.: 221,709

[30] Foreign Application Priority Data Dec. 17, 1968 U.S.S.R. 1291005 Dec. 24, 1969 U.S.S.R. ..1388901 52] US. Cl ..317/3 ..B05b 5/00 Field of Search 1 7/3 51 Apr. 3, 1973 [56] References Cited UNITED STATES PATENTS 2,509,277 4/1950 Ransburg et al ....3 17/3 2,767,359 10/1956 Larsen et al ..3l7/3 Primary Examiner-L. T. Hix Attorney-Holman & Stern [57] ABSTRACT A protective device preventing the transformation of a corona discharge into a spark discharge in electrical apparatus, particularly, in electrostatic painting apparatus, consists of a multielectrode gas discharge tube having a control grid which through a starting unit is connected to pickups or sensing elements controlling the operating current. The multielectrode gas discharge tube which may be a thyratron has a keeplive electrode. A comparison unit having a source of reference voltage is connected through a contact of a time relay to the keep-live electrode.

6 Claims, 3 Drawing Figures I PATEt-HEUAFRIs 191s SHEET 1 BF 3 Q :2 V a Q .HIIIIMHHH T g PATEhTED i I973 SHEET 2 BF 3 DEVICE FOR PREVENTING A CORONA DISCHARGE FROM ASSUMING THE FORM OF A SPARK DISCHARGE IN ELECTROSTATIC PAINTING APPARATUS BACKGROUND OF THE INVENTION spark discharge phenomena and, more particularly, to

devices for preventing a corona discharge from assuming the form of a spark discharge in electrostatic painting apparatus.

DESCRIPTION OF PRIOR ART At present, much attention is being given to the quality of varnish-and-paint coatings of various articles from the viewpoint of their corrosion resistance and their appearance to make these articles competitive on the market.

Electrostatic painting is known to be one of the most advanced and economically expedient technological processes. Thus, replacement of pneumatic sprayers of paint by electrostatic sprayers saves a large amount of varnish-and-paint materials and allows one to reduce the number of manual operations involved and overall maintenance costs, as well as the consumption of compressed and suction air, heat, etc.

A known electrostatic painting apparatus comprises a high-voltage rectifier with a grounded positive terminal, and a negative terminal connected to sprayers for applying the varnish-and-paint coating on an article or a workpiece. The article to be painted is positioned in front of the sprayers and is connected to the ground wire of the apparatus. The end face of the sprayer and the article form a discharge gap. When the apparatus is connected to a supply line, a corona discharge charge appears at the edge of the sprayer assisting in intensive electric charging of the aerosols of the varnish-andpaint material transferred to the article by electrostatic forces.

If the gap between the article and the sprayer is reduced during operation of the apparatus, this may result in transformation of the corona discharge into a spark discharge larcingl. There is a potential hazard in that since the components of the varnish-and-paint materials have a low ignition point, they are liable to catch fire.

To avoid spontaneous ignition, the discharge gap is usually made greater, and varnish-and-paint materials are selected to have a high ignition point. However, this is associated with low efficiency of the electrostatic painting apparatus and poor quality of the coating.

A high quality of the paint coating applied onto an article in an electrostatic field can, assuredly be obtained only by preventing discharge in the gap between the article and the sprayer, from taking the form of an arc.

The spark protection of the discharge gap is presently achieved by the use of various devices. Known in the art are passive devices to limit the rectifier current. Such a passive device for instance includes a rectifier-sprayer, a resistor or an inductive device connected in series with the rectifier circuit as well as the sprayers themselves, when made of semiconductor materials, serve as a resistor.

Also such a method of limiting the current in the working circuit reduces the intensity of the discharge and the efiiciency of the electrostatic painting apparatus.

Also known in the art are electromechanical and electronic active devices employed for the same purpose and comprising circuit breakers connected in series to the ground circuit of a high-voltage rectifier. These devices are operated by extra currents appearing in the circuits when a spark discharge is formed in the discharge gap. The principal disadvantage of such a circuit breaker consists in that it operates and deenergizes the rectifier only after the spark has appeared. What is more, the device takes a considerable time for its operation (e.g., tens of microseconds) and does not provide for discharging the spurious reactances formed by the windings of the rectifier transformer, the wiring and the discharge gap where electric energy has accumulated.

The best devices for protection of the discharge gap against sparking are those which prevent formation of sparks. The principle of operation of these devices consists in bridging the discharge gap by elements capable of passing therethrough the total current of the electrostatic painting apparatus under emergency operating conditions.

The elements for bridging the discharge gap are made in the form of spark discharge devices consisting of two ball-shaped electrodes disposed at a double breakdown distance from each other and fired by an additional third electrode which is fed with a voltage pulse whose intensity is sufficient for a breakdown between the additional electrode and one of the balls. These discharging devices however take a considerable time for electrical breakdown between the ball shaped electrodes and the additional electrode which sometimes exceeds the time of forming a spark discharge in the discharge gap.

Vacuum tubes being practically non-inertia devices are also used for bridging the discharge gap. However, these devices feature a high internal resistance and consume considerable power which may sometimes be higher than the total power of the electrostatic painting apparatus.

Also known in the art are gas discharge ionic devices which are most suitable for the above-mentioned purpose. They have very low internal resistance and, there fore, feature a very low voltage drop across their terminals. This is an advantage of such devices since the same low voltage (termed residual voltage) is applied across the discharge gap. Thus, in the case of a spark discharger the residual voltage is equal to tens of kilovolts, in the case of vacuum tubes it is equal to several kilovolts, whereas in the case of a gas discharge device such as for instance a thyratron this voltage is equal to tens of volts.

Known in the art is such a device for preventing an electric breakdown in the discharge gap of a painting apparatus employing a thyratron (USSR Inventors Certificate No. 122052).

The known device is characterized in that it is parallel connected to the high-voltage winding of a transformer associated with a rectifier further the thyratron used is a multielectrode type having a relay in its anode circuit, the contacts of the relay being inserted into the primary circuit of the transformer. The thyratron is rendered conductive with a voltage increase across a resistance connected into the high-voltage winding of the transformer.

In this device, a sensing element (or a pickup) responding to changes in the operating current uses a vacuum tube (e.g., a pentode) with a cathode resistor connected in series into the rectifier-sprayer circuit. Under emergency conditions caused by a decrease in the discharge gap between the sprayer and the article being painted, the operating current of the apparatus drastically increases. In this case the voltage drop between the anode and the cathode of the pentode also increases, the thyratron is rendered conductive and bridges the discharge gap.

The above described device has however the following disadvantages. It has no circuit which would keep the thyratron non-conductive under normal operating conditions, and this considerably reduces the operational reliability of the device itself because any occasional fluctuation in the voltage of the high-voltage source may render the thyratron conductive and interrupt the process of painting. In addition, series connection of the pentode pickup in the circuit being controlled results in that this device can control the circuit of one sprayer only. An increase inthe number of sprayers considerably reduces the sensitivity of the pickup and the efficiency of the device as a whole. Therefore, use of such a device reduces the efficiency of the electrostatic painting apparatus.

Further, since a thyratron has a limited life and operates as a driven element, the absence of a circuit for controlling the operating conditions of the thyratron causes breaks in the technological process which would be necessary for checking the operation of the thyratron.

Moreover, the development of a discharge in a multigrid thyratron takes a time of 1.5 to 2 microseconds and this being commensurate with the time of forming a breakdown in the discharge gap may accordingly cause a spark discharge.

DESCRIPTION OF THE INVENTION An object of the present invention is to provide a device for controlling the corona discharge within the discharge gap and to prevent this discharge from assuming the form of a spark discharge in an electrostatic painting apparatus, to reduce the time of driving the device into operation and to ensure automatic control of the thyratron operation.

A specific object of the invention is to provide a novel circuitry of the device which will widen the field of its application.

These and other objects are attained by providing a device for preventing corona discharge in an electrostatic painting apparatus from assuming the form of a spark discharge, said device comprising a high-voltage multielectrode gas discharge tube, pickups or sensing elements controlling the operating current in the articles being painted and a power pack; the device, according to the invention, has also a time relay, a comparison unit connected to the winding of an output relay, a reference voltage source, a separation transformer and a driving unit to drive the gas discharge tube into conduction, the control grid of said gas discharge tube being connected to said pickups controlling the operating current through said driving unit while a keep-alive electrode of the gas discharge tube is connected to the comparison unit through said time relay, the contacts of the output relay being connected in parallel to the secondary winding of the power pack separation transformer.

This device was subjected to industrial tests over a length of time and was found a highly efficient and reliable system. It has been found that this device makes possible to use of such paints as comprising perchlorovinyl and nitrocellulose. Tests have proved that the device can operate at voltages of -140 kV controlling the operation of 1-6 sprayers at operating currents of 20-500 microamperes. The bridging cur rent passing through the thyratron is equal to 20-25 amperes. The overall dimensions of the device in the cited example X 425 X 335 mm. The device has a cantilever construction with a high-voltage separation transformer serving as a base. Mounted on the transformer is an electronic unit comprising pickups or sensing elements, a driving circuit, a control circuit and a high-voltage thyratron. The pickups are mounted on knife connectors of the unit.

DESCRIPTION OF PREFERRED EMBODIMENT The invention will be better understood from the following detailed description of an embodiment thereof with reference to the accompanying drawings, wherein a specified terminology is used in the description. It should be noted, however, that the invention is not limited by the narrow terminology used, and the invention includes all the equivalent elements and units operating under similar conditions and used for performing given functions.

BRIEF DESCRIPTION OF THE DRAWING Other objects and inherent advantages of the present invention will be apparent from the following detailed description of the invention with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a device for preventing the corona from assuming the form of a spark discharge in an electrostatic painting apparatus;

FIG. 2 is a preferred exemplary schematic diagram of the device, and

FIG. 3 is a general view of the device.

Referring now to the drawings, pickups 3a to 3i inserted into the branched circuit of sprayers la to 1i (FIG. 1), connected to a high-voltage rectifier 2. The outputs of the pickups are associated with the inputs of a driving unit 4 whose output is connected to a control grid 5 of a thyratron 6. A keep-alive electrode 7 of the thyratron 6 is connected to a unit 8 for controlling the operation of the thyratron 6. This unit 8 consists of a time relay 9, a comparison unit 10, a reference voltage source 11 and a relay l2. Normally closed contacts 13 of the time relay 9 are inserted between the input of the comparison unit 10 and the keep-alive electrode 7. Normally open contacts 14 (FIG. 2) of the relay 12 are connected in series with a limiting resistor 15 and bridge the secondary winding 12 of a separation transformer 17. Connected to a primary winding 18 of the transformer 17 is a control-and-signalling unit 19. Normally closed contacts 20 of the unit 19 are connected to the input of the high-voltage rectifier 2. The input of a power pack 21 is connected to the secondary winding 16 of the transformer 17. The outputs of the power pack 21 are connected to the driving unit 4 as well as to the high-voltage thyratron 6 and the unit 8 for controlling the operation of the thyratron 6.

Each of the pickups 3a-3i (FIG. 2) is built around an R-C circuit one of the resistances of which is adjustable.

The driving unit 4 employs low-voltage thyratrons 22a 22i. The inputs of these stages are connected to the corresponding pickups 3a 3i, while the outputs of the same are connected to a common resistor 23. The resistor 23 is connected to the control grid 5 of the thyratron 6.

The time relay 9 of the unit 8 controlling the operation of the thyratron 6 is built around an RC circuit 24,25 which is connected to the input of a dynistor 26. The output of the dynistor 26 through a resistor 27 is connected to the base of a transistor 28 loaded with an electromagnetic relay 29. Through the contacts 13 of the electromagnetic relay 29 the comparison unit is connected to the keep-alive electrode 7 of the thyratron 6.

The comparison unit 10 uses a transistor 30 loaded with the winding of the electromagnetic relay 12. The reference voltage is provided by a Zener diode 11 inserted into the emitter circuit of the transistor 30.

The power pack 21 uses two transformers 31 and 32, the secondary windings 33, 34, 35 and 36 of the transformer 31 being connected to bridge rectifiers 37, 38, 39 and 40 respectively. Connected to the outputs of the bridge rectifiers are filters 41, 42, 43 and 44. A filament 46 of the thyratron 6 is connected to a portion of the primary winding of the transformer 32, the secondary winding 47 of the transformer 32 being connected to the filaments of the thyratrons 22a 22i. Primary windings 45 and 54 of the transformers 32 and 31 are connected to the secondary winding 16 of the separation transformer 17.

The control-and-signalling unit 19 employs two relays 48 and 52. The winding of the relay 48, its contacts 49 and push buttons 50 and 51 are connected to mains terminals 52 as a series-parallel circuit. The relay 53 serves as a signalling relay and is connected in parallel to the relay 48.

The separation transformer 17 (FIG. 3), serves as a base for mounting thereon a cabinet 55 of the device accommodating units and components 4,8 and 21 (the unit 19 is not shown in FIG. 3). The pickups are mounted on the rear wall of the cabinet 55.

The thyratron 6 enclosed into a pipe of an insulating material is mounted under the cabinet 55. Located on the front panel of the cabinet 55 are instruments to control the currents in the filament and keep-alive electrode circuits of the thyratron 6, control knobs, and a ball contact 56 to be connected to the wire from the rectifier 2. Each of the pickups 3a-3i is equipped with a similar ball contact for connection of the wires passing to the sprayers.

The control of the corona discharge and prevention of the corona-to-spark discharge phenomena are effected as follows. Let us assume that one of the articles 57a-57 being painted (FIGS. 1,2), for any reason, changes its position so that the discharge gap between the sprayers 1 and the article 57 decreases to a sparking distance. In this case the operating current in the corresponding pickup increases and the pickup sends a signal to operate the driving unit 4 which, in turn, sends a signal to the control grid 5 of the thyratron 6 resulting in its ignition. The ignited thyratron 6 short-circuits the rectifier 2 and bridges the discharge gap between the sprayer 1 and the article 57. The current in the circuit of the rectifier 2 drastically increases and causes the operation of its own protective circuit whose contacts 58 are connected in series with the contacts 20. The electrostatic painting apparatus is therefore deenergized.

The operation of the thyratron 6 is controlled as follows.

When the electrostatic painting apparatus and the device for preventing the corona discharge from assuming the form of an are are energized again by depressing the push button 50 (FIG. 2), the time relay 9 (FIG. 1) operates and starts time metering. The contacts 13 of this relay break and disconnect the comparison unit 10 from the circuit of the keep-alive electrode 7 until its conditions correspond to the operating duty. After a certain period of time determined by the value of the filament current of the thyratron 6, the time relay is deenergized and its contacts 13 connect the comparison unit 10 to the keep-alive electrode 7.

Assume that the cathode emission of the thyratron 6 falls down, or gas penetrates into the thyratron bulb, or the filament voltage drops, etc. In this case, the current of the keep-alive electrode 7 decreases, while the voltage increases. The comparison unit 10 comparing this voltage with the reference voltage of the source 11 operates at a definite ratio of these voltage. The signal from the comparison unit 10 triggers the electromagnetic relay 12 which, through its contacts 14, bridges the secondary winding 16 of the transformer 17. The extra currents in the primary winding 18 of the transformer 17 cause operation of the control-and-signalling unit 19 and disconnect the device from the power mains through the contacts 49. At the same time, the electrostatic painting apparatus is deenergized by the contacts 20 of the relay 53, the latter also sending an alarm signal.

What is claimed is:

1. A device for preventing a corona discharge from assuming the form of a spark discharge in an electrostatic painting apparatus for painting articles, comprising: a multielectrode high-voltage gas discharge tube; sensing means acting as pickups for controlling the operating current flowing through the articles being painted; and a power pack, the device characterized in that it include a time relay; a comparison unit connected to a winding of an output relay having contacts, a source of reference voltage, a separation transformer including a secondary winding and a driving unit for the gas discharge tube, said gas discharge tube having a control grid connected to said pickups through said driving unit and a keep-alive electrode connected to said comparison unit through said time relay, the contacts of said output relay being connected in parallel with the secondary winding of the separation transformer of the power pack.

2. A device as claimed in claim 1 wherein each of said pickups comprises an R C circuit including an adjustable resistance.

of reference voltage comprises a Zener diode.

6. A device as claimed in claim 1 wherein said power pack comprises transformers having secondary windings connected to bridge rectifiers. 

1. A device for preventing a corona discharge from assuming the form of a spark discharge in an electrostatic painting apparatus for painting articles, comprising: a multielectrode high-voltage gas discharge tube; sensing means acting as pickups for controlling the operating current flowing through the articles being painted; and a power pack, the device characterized in that it include a time relay; a compariSon unit connected to a winding of an output relay having contacts, a source of reference voltage, a separation transformer including a secondary winding and a driving unit for the gas discharge tube, said gas discharge tube having a control grid connected to said pickups through said driving unit and a keep-alive electrode connected to said comparison unit through said time relay, the contacts of said output relay being connected in parallel with the secondary winding of the separation transformer of the power pack.
 2. A device as claimed in claim 1 wherein each of said pickups comprises an R C circuit including an adjustable resistance.
 3. A device as claimed in claim 2 wherein said driving unit comprises a plurality of low voltage thyratrons.
 4. A device as claimed in claim 1 wherein said time relay comprises an R C circuit connected to the input of a dynistor.
 5. A device as claimed in claim 1 wherein said source of reference voltage comprises a Zener diode.
 6. A device as claimed in claim 1 wherein said power pack comprises transformers having secondary windings connected to bridge rectifiers. 